We present the third in a series of results from our programme of the detection and measurement of magnetic fields in protoplanetary nebulae (PPN), using full-polarization MERLIN observations of OH masers. These are the first maps of the OH 1612- and 1667-MHz masing emission from IRAS 20406+2953. We have identified a Zeeman pair in the 1612-MHz data; this result heralds the second magnetic field strength measured in a PPN. We show that the field, of strength −3.1 mG, is sufficiently strong to be shaping the outflowing mass from the star. By considering the spatial distribution of the masers and the structure of the linear polarization data, we suggest a field configuration that is toroidal. The observed change in polarization position angle between the peak red- and blueshifted components of the maser features, if attributed to Faraday rotation within the circumstellar envelope, gives an electron density of ∼1 cm−3. We show that this is sufficient to freeze the field into the outflowing wind.